Every day, crowds flock to the Lukács Thermal Baths in Budapest, soaking in warm mineral-rich pools as yellow trams clatter along Frankel Leó Street. Most never suspect that just yards away, beneath the city’s historic streets, lies a hidden world: a vast underwater cave system heated by geothermal springs.

From its entrance, tucked into the base of Rózsadomb — Rose Hill — an affluent neighborhood of elegant villas and tree-lined streets, the Molnár János Cave stretches for over 3.6 miles (5.8 kilometers) and plunges nearly 300 feet (90 meters) below the surface. Flooded with crystal-clear water at the temperature of a warm bath, it is one of the largest active thermal water caves in the world, and among the rare few open to certified cave divers.

The way into the cave is easy to miss fr

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